1. Field of the Invention
The present invention relates generally to an audio power amplifier for amplifying an audio signal to drive a speaker, and particularly to a digital power amplifier capable of amplifying an audio signal by driving a switching means with a signal modulated by a suitable modulation system such as a PWM (pulse width modulation) system.
2. Description of the Related Art
Heretofore, various power amplifiers for amplifying an audio signal to drive a speaker have become commercially available on the market. As one system of these power amplifiers, there have been developed a power amplifier called a digital power amplifier capable of amplifying an audio signal by directly switching a stabilized power-supply with a pulse signal modulated by an inputted digital audio signal. This digital power amplifier generates pulse-width-modulated (PWM) PWM wave (hereinafter this PWM wave will be referred to as a xe2x80x9cPWM signalxe2x80x9d) based upon the inputted digital audio signal, for example, and switches a stabilized DC (direct current) power-supply at a high speed by using this PWM signal to thereby obtain a speaker drive signal by extracting an audio signal component from the power-supply thus switched.
FIG. 1 of the accompanying drawings is a schematic block diagram showing an example of an arrangement of a digital power amplifier according to the related art. FIG. 1 shows a circuit arrangement of a related-art digital power amplifier capable of switching a power switch by using a PWM signal. As shown in FIG. 1, this digital power amplifier generates a PWM signal, which is a pulse-width-modulated pulse signal, based upon an audio signal supplied to a PWM signal generating circuit 91. A PWM signal outputted from the PWM signal generating circuit 91 is supplied to a switching module 92 which serves as a means for switching a power-supply.
As shown in FIG. 1, within the switching module 92, the supplied PWM signal is supplied through a buffer amplifier 93 to the gate of a first switching element 95. Further, the supplied PWM signal is supplied through an inverter gate 94 to the gate of a second switching element 96. The first and second switching elements 95 and 96 are each formed of a MOS (metal oxide semiconductor) field-effect transistor in which the source-drain path is conducted under control of the PWM signal developed at the gate thereof.
The first and second switching elements 95 and 96 are formed as means for switching a DC power-supply output from the power-supply circuit 97. Specifically, there is prepared the power-supply circuit 97 which generates a stabilized DC voltage by rectifying and smoothing a commercially-available power source. An output end of a positive (+) power source obtained from this power-supply circuit 97 is connected to the source of the first switching element 95, and an output end of a negative (xe2x88x92) power source obtained from this power-supply circuit 97 is connected to the drain of the second switching element 96. Then, the drain of the first switching element 95 and the source of the second switching element 96 are connected together to provide a junction 100. A signal developed at this junction 100 is supplied to a low-pass filter (LPF) 98 as an output from the switching module 92.
The low-pass filter 98 removes high-frequency components from the signals obtained at the switching of the first and second switching elements 95 and 96 to extract an audio signal component. The audio signal thus extracted is supplied to a speaker apparatus 99 as an output audio signal of an audio amplifier and thereby outputted from the speaker apparatus 99 to the outside.
The first and second switching elements 95 and 96 respectively function as switches which are driven in a push-pull circuit fashion. Specifically, the first and second switching elements 95 and 96 are each switching means in which the source-drain path conducts when a pulse waveform of a PWM signal supplied to the gate is held at high level and in which the source-drain path does not conduct when the above pulse waveform of the PWM signal is held at low level. The pulse waveform of the PWM signal supplied to the gate of the first switching element 95 and the pulse waveform of the PWM signal supplied to the second switching element 96 are in 180xc2x0 phase relationship with each other.
Therefore, when the pulse waveform supplied to the gate of the first switching element 95, for example, is held at high level, the pulse waveform supplied to the gate of the second switching element 96 goes to low level. As a consequence, any one of the positive power source and the negative power source is connected to and supplied to the side of the low-pass filter 98 connected to the junction 100 between the first and second switching elements 95 and 96 in response to the switching state obtained at that time. The switching is controlled as described above, whereby an audio signal waveform amplified by the voltage is outputted as the output of the low-pass filter 98. At that very moment, the central level of the audio signal waveform is set to 0 V.
The processing for generating a PWM signal, which is a pulse-width-modulated signal, from the audio signal is executed based upon a principle shown in FIGS. 2A to 2C, for example. When there is an analog audio signal having a sine wave shown in FIG. 2A, this analog audio signal is converted into digital data of one bit system. According to this embodiment, the analog audio signal is converted into digital data of one bit system, shown in FIG. 2B, in which the levels of the signal waveforms are expressed in the form of a pulse waveform density in a so-called DSD (direct stream digital) fashion. Then, based upon the digital data of one bit system, there is executed a processing for generating a PWM signal which is pulse-width-modulated as shown in FIG. 2C. Then, the switching means which switches the power-supply is turned on and off under control of the PWM signal thus generated with the result that there can be generated a waveform equal to the waveform of the analog audio signal amplified by the power-supply voltage.
When the power amplifier has the arrangement in which the two switching elements 95 and 96 are driven in a push-pull fashion as shown in FIG. 1, the inverted signal of the PWM signal which drives one switching element is generated by using the inverter gate 94 within the switching module 92. However, when the inverted signal is generated by using the inverter gate as described above, a very small difference occurs between a timing of the control signal supplied to the gate of one switching element and a timing of the control signal supplied to the gate of the other switching element. As a consequence, there arises a problem that noise will be generated from the power-supply due to the above difference between the timings.
FIGS. 3A through 3F are diagrams of waveforms of respective signals and respective differential components obtained when the two switching elements 95 and 96 are driven in a push-pull fashion in the circuit arrangement shown in FIG. 1. For example, when the PWM signal supplied from the PWM signal generating circuit 91 to the switching module 92 has a waveform shown in FIG. 3A, this pulse waveform is supplied to and amplified by the buffer amplifier 93 so that this waveform is slightly delayed from the waveform of the inputted PWM signal as shown in FIG. 3B. Further, the waveform of the inverted pulse which results from inverting the PWM signal inputted to the switching module 92 by the inverter gate 94 becomes a waveform which is slightly delayed from the inputted PWM waveform in timing as shown in FIG. 3C.
Since the buffer amplifier 93 and the inverter gate 94 are the circuits whose characteristics are different from a principle standpoint, a time difference occurs in the timings between the leading edge and the trailing edge of the control signal waveform (see FIG. 3B) of the first switching element 95 and the control signal waveform (see FIG. 3C) of the second switching element 96.
Accordingly, a time differential (first differential component) of currents consumed by the first switching element 95 occurs in the leading edge and the trailing edge of the pulse waveform of the signal supplied to the gate of the first switching element 95 as shown in FIG. 3D. Further, a time differential (second differential component) of currents consumed by the second switching element 96 occurs in the leading edge and the trailing edge of the pulse waveform of the signal supplied to the gate of the second switching element 96 as shown in FIG. 3E. As a consequence, the differential components are generated in the two switching elements 95 and 96 at different timings. Accordingly, the waveform of the signal radiated from the switching module 92 becomes a waveform upon which the respective differential components are superimposed as shown in FIG. 3F. Then, a signal corresponding to this differential waveform shown in FIG. 3F is generated from the transmission line of the PWM signal as the radiation, which as a result causes noise of the power-source to occur in each switching element. Consequently, it is unavoidable that audio characteristics of the audio signal outputted from the speaker apparatus 99 connected to the digital power amplifier are deteriorated, influenced by the noise.
In view of the aforesaid aspect, it is an object of the present invention to provide a digital power amplifier in which outputted audio characteristics can be prevented from being deteriorated due to influences exerted by noise from a power-supply.
According to an aspect of the present invention, there is provided a digital power amplifier which is comprised of a switching control means for generating first and second switching control signals which are inverted in phase with each other at an equal timing based upon an inputted audio signal, a first switching means which conducts/does not conduct under control of the first switching control signal and a second switching means which conducts/does not conduct under control of the second switching control signal, wherein an output signal is obtained from a junction between the first and second switching means.
In accordance with another aspect of the present invention, there is provided a digital power amplifier which is comprised of a switching control means for generating first and second switching control signals which are inverted in phase with each other at an equal timing based upon an input audio signal, a first switching means which conducts/does not conduct under control of the first switching control signal, a second switching means which conducts/does not conduct under control of the second switching control signal, a third switching means which conducts/does not conduct under control of the first switching control signal and a fourth switching means which conducts/does not conduct under control of the second switching control signal, wherein output signals are generated from a junction between the first and second switching means and a junction between the third and fourth switching means.
According to the present invention, since a timing at which one of the two switching means which are driven in a push-pull fashion conducts and a timing at which the other of the two switching means does not conduct become equal to each other and the time differentials of the currents consumed by the respective switching means are generated with opposite phases at an equal timing, the differential components are canceled each other out as a total current of the two switching means